Small-sized planing boats such as various types of fishing boats and pleasure boats are used for fishing, while they are moored on a water surface of the sea or lakes. Under the circumstances, preventing the boat from rocking when moored has been an issue to address. Patent Literature 1 discloses a hull stabilizing mechanism (anti-rocking device) including a rudder blade and a rudder blade moving mechanism, in which placing the rudder blade into water prevents the boat from rocking while removing the rudder blade from the water when sailing prevents the rudder blade from producing sailing resistance.